An engine in a vehicle creates noises during running of the vehicle. To improve comfort for passengers in the vehicle, it is common to incorporate an insulating device or noise reduction device in an engine compartment to reduce the noises caused by vibration of the engine. A conventional insulator consists of a plurality of layers, and the layer contacting the engine hood is fixed under the lower surface of the engine hood via adhesives or rivets. It can be difficult to remove the insulator from the engine hood, and thus not convenient for maintenance and replacement. Another type of conventional insulator is removably connected to an inner panel of the engine hood via a plurality of plastic clips. In order to assemble the plastic clips, the insulator may be formed with mounting holes. Chinese patent application No. CN201646598U discloses an engine hood insulator for a vehicle which defines a number of through holes for mounting several other parts of the vehicle. On the one hand, the assembly of the insulator may require a number of clips and thus will cause high production cost. On the other hand, the through holes not only reduce an insulating area of the insulator which can affect its insulation performance, but also create an uneven thickness of the insulator which can affect its appearance.